This invention relates to medical apparatus for the measurement, independent of gas type, of respiratory flow, having a breathing tube and a flow measurement sensor accommodated therein, for example a Fleisch pneumotachograph, a flow area restricting differential pressure type flowmeter, a thermal flowmeter, or the like.
Known devices for respiratory flow measurement operate, for example, with Fleisch pneumotachographs as respiratory flow receptors, such receptors being based on the principle that respiratory flow is subjected in the receptor to an effective flow resistance, the flow pressure being measured at two points, that is before and after the flow resistance. The pressure difference is proportional to the volume flow rate V, with a proportionality factor containing geometrical apparatus constants and, further, parameters dependent on the gas type of the respiratory medium, for example the viscosity .mu.. As a generalization, the signal output (S) from the usual flow measurement sensors can be expressed as a product of two functions, a flow function f(V) and a parameter function g(m.sub.i), that is: S=f(V) .multidot.g(m.sub.i), where m.sub.i for example signifies one of the gas parameters: viscosity .mu., density .rho. and temperature conductivity .lambda.. Other parameters are possible.
Because of the effect of the gas parameters, the range of application of the subject type of measurement apparatus has been limited. To be sure, the respiratory flow receptors can, by means of calibration with standard flows of a gas or also of a gas mixture of known composition, for example room air, be used for the measurement of the flow of the same respiratory gases with known composition. Such receptors are less suitable for the respiratory flow measurement of gases and gas mixtures of arbitrary or variable composition. The gas parameters must then, if necessary, be separately determined in special devices or be calculated for the respiratory gas mixture after a quantitative gas analysis, which can be very expensive. Thereafter, the result of the flow measurement must be corrected in accordance with the ascertained gas parameters. For the measurement of respiratory flow in connection with the evaluation of lung function by known techniques, such procedures are too expensive, considering the required exactness and speed of the measurement. It would indeed be desirable to carry out respiratory flow measurements with various respiratory gases at variable temperatures and partial pressures.